Field of the Invention
The present invention relates to a digital modulation apparatus, and in particular, to a modulator such as a quadrature phase shift keying (QPSK) modulator implementing, by use of digital signal processing means, a circuit in which a phase of a carrier is shifted by an integral multiple of .pi./2 according to an input signal, thereby accomplishing a modulation.
Description of the Prior Art
A system such as a QPSK modulator, having a phase modulator modulating orthogonal carrier waves by data to be transmitted, is configured by the digital technology which performs waveform shaping and an analog circuit which has been commonly used to modulate the carrier waves. In addition, a circuit modulating a carrier in a digital fashion is also known; however, as described in the Japanese Patent Unexamined Publication No. 53-24763 (Data Transmission System), a plurality of ROM's and adders are required, namely, the complexity of the circuit structure was not considered.